1. Field of the Invention
This invention relates to amplifiers and, more particularly, to a calibration system for a feed forward distortion reduction system.
2. Description of Related Art
Amplifiers often add undesired distortion to a signal, creating an output signal comprising distortion or nonlinear components and the input signal component. The distortion includes any undesired signals added to or affecting adversely the signal. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by the amplifier.
Feed-forward correction is routinely deployed in modern amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate distortion, such as intermodulation (IMD) components, created by the amplifier so that at the final summing point, the distortion cancels out. Due to the unpredictability of the input signal pattern as well as the resultant distortion location, some feed forward schemes inject a known signal, i.e. a pilot signal, in the main signal path to mimic the distortion produced by the amplification process. In feed-forward amplifiers, the feed forward distortion reduction circuitry minimizes the pilot along with the distortion. As such, by designing the feed forward distortion reduction circuitry to detect and cancel the pilot, the distortion can also be removed.
The output of the feed forward amplifier system contains the reduced pilot signal which is detected using detection circuitry to provide information to the feed forward distortion reduction circuitry to improve cancellation of the pilot signal at the output. The information obtained from detecting the pilot signal at the output of the feed forward distortion reduction circuitry can also compensate for changes in the operation of individual components of the feed forward distortion reduction circuitry. In actual systems, however, there is rarely an absolute cancellation of the distortion and the pilot signals. Feed forward distortion reduction systems require tight operating tolerances, for example, to achieve a 30 dB reduction in IMDs, typical feed forward correction systems may require a + or -0.1 dB frequency flat response (amplitude deviation over the frequency band of operation) and a + or -1 degree phase linearity (phase deviation from a straight line in the frequency band of operation). To obtain this accuracy is difficult.
The amplitude of the pilot signal is typically relatively small at the output of the feed forward distortion reduction system because of the cancellation of the pilot and the relative amplitude of the pilot signal with respect to the amplitude of the output signal. Thus, it becomes difficult to detect the pilot signal at the output of the system. To improve detection of the pilot signal at the output of the correction circuitry, schemes are developed to place the pilot signal at an appropriate location and to improve detection.
Pilotless feed forward distortion reduction schemes have been developed to eliminate the need for a pilot signal. The pilotless feed forward reduction schemes, however, do not have a known pilot signal which can be detected at the output of the feed forward distortion reduction system which can be used to compensate for changes in the operation of the feed forward reduction circuitry. As such, pilotless feed forward reduction circuitry can degrade in performance as individual components exhibit changing operating characteristics, for example, due to aging or temperature change.
A need exists for a calibration system for feed forward distortion reduction systems to improve distortion reduction.